Devices using new materials taking the place of silicon, such as silicon carbide (SiC) and nitride semiconductors, are developed these days.
As an example, it is known that if a hetero-junction in which gallium nitride (GaN), which is a nitride semiconductor, and an aluminum gallium nitride (AlGaN) are stacked is formed, two-dimensional electron gas (2DEG) is generated at the interface of the hetero-junction. A hetero-structure field effect transistor (HFET) unitizing the 2DEG as a channel has the properties of high breakdown voltage and low ON resistance.
Here, as a structure to achieve normally OFF operation in the GaN-based HFET, there is a structure in which a recess structure is formed and a gate electrode is formed via an insulating film. There is a need for further improvement to obtain low ON resistance, high breakdown voltage, and high reliability in such a nitride semiconductor device.